Count sb in: To include someone in an activity or arrangement; to consider (someone) as one of the people who will be doing something. As in Do you want to go swimming with us tomorrow? Yes, count me in.
中文释义：count意思是算数，count sb in把某人算在内。网络流行语经常使用+1来表示我要加入、算我一份，看到朋友在群里组团出去玩、拼团抢购好物，这时没有喊个+1多不合群，count me in=+1。看到一群人在兴高采烈地狂欢，如果你也想加入，就大喊一句count me in吧。例句：例：Do you want to go to the beach with us? Yes! Count me in!（想跟我们一起去海边么？想！算我一个！）